Method for producing a ceramic substrate

ABSTRACT

A method is disclosed for producing a ceramic substrate made of base that includes a stack of layers. Each layer in the stack includes a non-sintered ceramic material and a binder. The method includes debinding the layers in a temperature interval of T E1 -T E3 , where T E1  is a minimum debinding temperature and T E3 &gt;T E1 , and sintering the layers at a temperature T S , where T S ≧T E3 . Debinding and sintering are performed in a same furnace, and a temperature T of the base does not fall below T E1  during debinding and sintering.

TECHNICAL FIELD

The invention relates to a method for the production of a ceramicsubstrate comprising a stack of layers stacked on top of one another.

Each of the layers of the stack contains a non-sintered ceramic materialand binder, together forming a basic body, which is debinded andsintered.

BACKGROUND

A method is known in which, for example, the decarbonization of thebasic body is performed in a batch furnace, which is heated over anextend period of time to a temperature suitable for decarbonization.Then the basic body, cooled to room temperature, is removed from thebatch furnace and, together with other already decarbonized basicbodies, placed onto a conveyer belt and fed into a sintering furnaceconfigured for conveyor belts. The use of a continuous process isfeasible here, because the sintering is not as time-consuming as thedecarbonization.

A disadvantage of the known method is that it is unsuitable for theproduction of multi-layer substrates made of various ceramic materialshaving different relative permittivities and generally differentcoefficients of thermal expansion, in which layers with feedthroughs areprovided. It was discovered that during the production of suchmulti-layer substrates, cracks often occur between the feedthroughs,which are normally made of a metalliferous paste, and the ceramic layerforming the surrounding environment of the feedthrough.

SUMMARY

It is, therefore, the goal of the present invention to provide a methodfor the production of a ceramic multi-layer substrate, in which the riskof crack development is reduced. This goal is achieved by means ofmethod according to claim 1. Advantageous further developments of theinvention are found in the additional claims.

The invention indicates a method for the production of a ceramicsubstrate, in which a basic body is made available in a first step. Thebasic body contains a stack of layers stacked on top of one another. Thestacked layers each consist of a non-sintered ceramic material, whichalso contains a binder. The basic body contains electrically conductivevertical ducts, each of which passes through at least one of the layers.In a further step, the layers are preferably debinded in an inertatmosphere (e.g., in an atmosphere containing nitrogen), a minimumtemperature being maintained during debinding. In the ensuing step, thebasic body is sintered, the sintering taking place at a temperaturewhich is greater that the minimum temperature of debinding. The minimumtemperature of debinding is dependent upon the composition of thematerial to be debinded. Throughout the entire of the two process stepsmentioned last, the temperature T of the basic body is maintained at alevel that does not fall below the minimum temperature T_(E1) of thedebinding.

According to the invention, both debinding and sintering are performedin one and the same furnace. This prevents the ceramic basic body fromcooling to room temperature. In addition, this simplifies the method,because switching furnaces is avoided.

An advantage of the method is that it is possible to avoid cooling thebasic body to room temperature between debinding and sintering. This isbecause cooling the basic body to room temperature between debinding andsintering holds the risk that, because of the different coefficients ofthermal expansion of the electrically conductive duct and thesurrounding layer, cracks can occur. As a result of the method, coolingto room temperature is avoided, also making it possible to reduce therisk of crack development. The basic body is only cooled aftersintering, that is, during the stage in which the ceramic materials arealready sufficiently stable, thereby reducing the risk of crackdevelopment.

Debinding is defined as a process suitable for volatilizing the organiccomponents, especially solvents and binders, from the layers.

It is possible to perform the process steps of debinding and sinteringin an air atmosphere. It is also possible to perform debinding andsintering in an inert atmosphere. Another possibility is to convert theatmosphere in the furnace during debinding in order, for example, tocontrol the rate of oxidation of the organic components. Anotherpossible means of controlling the rate of oxidation is a correspondingadjustment of the furnace's temperature program, in which the directionand rate of temperature change is advantageously variable. For example,the temperature can be kept constant or reduced in some time ranges,although the current temperature must always remain above the minimumtemperature of debinding.

According to the invention, debinding is performed within a temperaturerange of T_(E1)-T_(E3) (T_(E1)<T_(E3)), wherein the temperature in onevariant of the invention can be increased, essentially monotonically,from T_(E1) to T_(E3) during debinding.

In the preferred variant of the invention, the temperature is initiallyincreased, preferably monotonically, from T_(E1) to T_(E2)(T_(E1)<T_(E2)<T_(E3)). This step is preferably performed in an inertatmosphere, wherein unwanted oxidation of the solvents and binders canbe intentionally (temporarily) reduced and/or prevented.

Subsequently, the atmosphere in the furnace is converted to airatmosphere. If the furnace atmosphere is converted at relatively hightemperatures>T_(E2), there is a risk that the oxidation of the organiccomponents will occur too quickly and that the carbon dioxide developingin the process will emerge explosively from the layers. To prevent therapid emergence of carbon dioxide from the layers, the temperature T ofthe basic body can be reduced, preferably concurrently with theconversion of the atmosphere or immediately thereafter, fromT_(E1)≦T_(E1′)<T_(E3). Then the temperature is preferably monotonicallyincreased to the final temperature T_(E3) of debinding.

This is followed by a further increase in the temperature to at leastthe value required to sinter all layers. Only after sintering is thebasic body and/or the multi-layer substrate cooled to room temperaturein the furnace and then removed from the furnace.

However, the cooling of the basic body to room temperature only occursin a stage in which the stability of the ceramic layers is alreadysufficient to reduce the risk of development of cracks.

Between the layers, structured metallization layers are preferablyprovided which, like the vertical electric ducts, can be produced usinga metalliferous paste.

The stack of layers stacked on top of one another is preferably a stackin which each of the layers containing ceramic material containsopenings, which are filled with a metalliferous paste. The advantage ofthis is that the electrically conductive connections between metallayers disposed on top of one another are easily established.

The layers containing non-sintered ceramic material, which are normallyalso referred to as green foils, can already be provided with openingsprior to formation of the stack. This can be achieved by means ofpunching, for example. Following the punching, the openings are filledwith a metalliferous paste. Only then are the green foils stacked on topof one another and is the basic body produced by means of lamination.

A paste containing precious metals, such as silver and palladium, can beused advantageously as a metalliferous paste.

It is also advantageous if the materials of layers stacked on top of oneanother in the stack are different and the stack, therefore, contains atleast two different ceramic materials. It is possible, for example, touse a ceramic material for a green foil disposed in the interior of thestack which has relative permittivity of approx. ε=20. This makes itpossible to produce ceramic substrates that contain high-capacitycapacitors. Furthermore, it is advantageous to produce the lowest andthe higher layer of the stack of layers stack on top of one another bymeans of a ceramic material with a lower relative permittivity of, forexample, ε=8.

Furthermore, it is advantageous to select the materials in accordancewith the following rule:

A first ceramic material contained in the basic body begins to sinter ata temperature T_(S1). A second ceramic material contained in the basicbody begins to sinter at a temperature T_(S3). Furthermore, themetalliferous paste contained in the basic body begins to sinter at atemp T_(S2). In addition, the following applies: T_(S1)<T_(S2)<T_(S3).To obtain a densely sintered basic body, it is preferable to sinter at atemperature that exceeds the sintering temperature T_(S3).

Advantageously, the following can apply to the relative permittivity ε₁of the ceramic material with the smaller relative permittivity:7≦ε₁≦8.5.

In addition, the following can apply to the relative permittivity ε₁ ofthe ceramic material with the larger relative permittivity: 18≦ε₂≦22.

A structured metallization layer made of the structured metalliferouspaste can be provided on the uppermost layer, below the lowest layers,and between two layers stacked on top of one another. The ductsconstitute vertical electrical connections between the metallizationlayers.

In the following, the invention is explained in greater detail, using anexemplary embodiment and the related figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, as an example, the temperature progression duringcompletion of the method according to the invention in a chart in whichthe temperature is plotted over time.

FIG. 2 shows, as an example, a substrate produced by means of the methodaccording to the invention, in schematic cross-section.

DETAILED DESCRIPTION

FIG. 1 shows, as an example, a temperature profile for the use of themethod according to the invention to produce a ceramic substrate. One ormore basic bodies are heated for a period of three hours from roomtemperature to a temperature T_(E1) (beginning of debinding). In theexample from FIG. 1, T_(E1)=200° C. This is the level below which thetemperature should not fall during the debinding. The increase in thetemperature to T_(E1) can occur in an air atmosphere or, alternatively,in an inert atmosphere.

Beginning at this temperature, a large fraction of the organiccomponents begins to escape. For this reason, the temperature ispreferably increased uniformly and slowly (e.g., for approx. 13 hours),in an inert atmosphere, to a value T_(E2). During the further process,the furnace, in this variant of the invention, is converted to an airatmosphere and, in this process, the temperature is initially reducedagain to a value of T_(E1)≦T<T_(E2). Without such a temperaturereduction, explosion-like deflagration can occur, which can lead todamaging or destruction of the ceramic basic body.

Following the conversion of the furnace to an air atmosphere, thedecarbonization process is continued. The temperature of the basic bodyis now increased to a temperature T_(E3), which is preferably 450° C.and is kept constant for a period of about one hour. At this point, thelast organic components of the basic body also escape. Decarbonizationis then complete. The ceramic basic body is subsequently heated, in aramp-like fashion, to the sintering temperature T_(S1), at which thelayers 2 of a first ceramic material begin to sinter.

A ceramic substrate in accordance with FIG. 2 is considered for theexemplary temperature profile from FIG. 1. The ceramic substratecomprises a basic body 1, which contains a stack 1 a of layers 2, 3stacked on top of one another. The layers 2, 3 stacked on top of oneanother contain a non-sintered ceramic material, which, in turn,contains additional organic components such as solvents and binders. Inthis connection, the layers 2 are preferably executed in such a way thatafter sintering their relative permittivity ε₁ is approx. 8. Anotherlayer 3 consisting of a second ceramic material (or at least a partiallayer made of the second ceramic material) is disposed between the twolayers 2. The second ceramic material is a ceramic material (preferablya so-called K20 material) with a relative permittivity ε₂ that differsfrom ε₁. The K20 material is a ceramic material with relativepermittivity ε₂ of approximately 20. In addition, vertical openings areformed in the layers and are filled with a metalliferous paste, therebyforming electrically conductive ducts 4. Internal electrodes 6 aredisposed in the individual plies of the layer 3 in such a way as to forma capacitor. The capacitor is connected, in electrically conductivefashion, with an electric component 5 on the upper side of the basicbody 1.

The sintering temperatures of the materials in the layers 2 and 3,and/or in the electrically conductive duct 4, are selected in such a waythat the following applies to the sintering temperature T_(S1) of theceramic material of layer 2, to the sintering temperature T_(S3) of theceramic material of layer 3, as well as to the sintering temperatureT_(S2) of the metalliferous paste: T_(S1)<T_(S2)<T_(S3). According toFIG. 1, the following applies to the temperatures:

-   -   T_(S1)=625° C.    -   T_(S3)=700° C.

From the above, it follows that after ramping up the temperature over anextended period of 40 hours, a temperature of 625° C. is reached, whichis the temperature at which the first ceramic material begins to sinter.The temperature is kept constant for about 2 hours, and is then quicklyincreased to the value T_(S)≧T_(S3), e.g., T_(s)=900° C. This brings thetemperature to a level at which the layers 2, 3 of the ceramic basicbody are densely sintered. At the same time, namely during thetemperature increase from T_(S1) to T_(S), the layer 3 of the secondceramic material, with a sintering temperature of T_(S3), as well as themetallic duct and the internal metal coating begin to sinter. Thistemperature is now held for 0.25 seconds to obtain a thickly sinteredceramic basic body.

Only after all layers and materials of the basic body are sintered isthe temperature gradually reduced to room temperature if applicable FIG.1.

At least one of the layers of the first ceramic material can form astratified compound with at least one of the layers of the secondceramic material, wherein several of such stratified compounds arepreferably formed and wherein each of the structured metallizationlayers is provided between the stratified compounds.

The present invention is not limited to the production of substratesthat contain K8 and/or K20 materials, but instead is applicable to alltypes of ceramic substrates that contain feedthroughs or electricallyducts.

1. A method of producing a ceramic substrate comprised of a base thatcomprises layers in a stack, each layer in the stack comprising anon-sintered ceramic material and a binder, the method comprising:debinding the layers in a temperature interval of T_(E1) to T_(E3),where T_(E1) is a minimum debinding temperature and T_(E3)>T_(E1); andsintering the layers at a temperature T_(S), where T_(S)≧T_(E3); whereindebinding and sintering are performed in a same furnace; wherein atemperature T of the base does not fall below T_(E1) during debindingand sintering; wherein debinding begins at a temperature between T_(E1)and T_(E2) that increases at an increasing rate, whereT_(E1)<T_(E2)<T_(E3), whereafter T decreases to a value of T_(E1′),where T_(E1)<T_(E1′)<T_(E2); wherein a first part of debinding isperformed in an atmosphere that is inert; and wherein, during debinding,an atmosphere in the furnace changes from an inert atmosphere to an airatmosphere in accordance with a reduction in temperature to T_(E1)′. 2.The method of claim 1, further comprising: forming the stack of layers;wherein forming comprises forming openings in the layers and adding ametalliferous paste to at least some of the openings.
 3. The method ofclaim 2, wherein the metalliferous paste comprises silver orsilver—palladium.
 4. The method of claim 2, wherein the stack of layerscomprises a first layer comprised of a first ceramic materials, and asecond layer comprised of a second ceramic material, the second layerbeing above the first layer; wherein the first ceramic material beginsto sinter at a temperature T_(S1), the second ceramic material begins tosinter at a temperature T_(S3), and the metalliferous paste begins tosinter at a temperature T_(S2); and wherein T_(S1)<T_(S2)<T_(S3).
 5. Themethod of claim 4, farther comprising: forming a stratified compoundusing the first layer and the second layer, the ceramic substratecomprising plural stratified compounds; and forming structuredmetallization layers between the stratified compounds.
 6. The method ofclaim 2, wherein forming comprises providing structured metallizationlayers between layers in the stack comprised of sintered ceramicmaterial, the structured metallization layers comprising themetalliferous paste.
 7. The method of claim 1, wherein at least two ofthe layers comprise different ceramic materials.
 8. The method of claim4, wherein, following sintering, the first ceramic material has arelative permittivity ε₁, where 7≦ε₁≦8.5; and wherein, followingsintering, the second ceramic material has a relative permittivity ε₂,where 18≦ε₂≦22.
 9. The method of claim 1, wherein T increases at asubstantially constant rate to a value T_(E3) after T decreases to thevalue of T_(E1′).